Activation of Ca2+-calmodulin kinase II induces desensitization by background light in dogfish retinal ‘on’ bipolar cells

Retinal ‘on’ bipolar cells possess a metabotropic glutamate receptor (mGluR6) linked to the control of a G-protein and cGMP-activated channels which functions to generate high synaptic amplification of rod signals under dark-adapted conditions. Desensitization of ‘on’ bipolar cells is initiated by a rise in Ca 2+ during background light too weak to adapt rod photoreceptors. Desensitization could also be elicited by raising intracellular Ca 2+ above 1 μ m . In order to investigate the mechanism of desensitization, whole-cell current responses to brief flashes and to steps of light were obtained from voltage-clamped ‘on’ bipolar cells in dark-adapted dogfish retinal slices. The inclusion of Ca 2+ -calmodulin kinase II (CaMKII) inhibitor peptides in the patch pipette solutions not only blocked desensitization of ‘on’ bipolar cells by dim background light and by 50 μ m Ca 2+ , but also increased their flash sensitivity. The substrate of phosphorylation by CaMKII is the ‘on’ bipolar cell cGMP-activated channels. Desensitization probably results from a reduction in their sensitivity to cGMP and a voltage-dependent decrease in their conductance. A role for protein kinase C (PKC) in this process was excluded since activating PKC independently of Ca 2+ with the phorbol ester PMA failed to induce desensitization of ‘on’ bipolar cells.